void mainImage(out vec4 fragColor, in vec2 fragCoord){
    vec2 uv=fragCoord/iResolution.xy;
    uv.x *= iResolution.x/iResolution.y;
    uv = (uv - .5) * 2.;
    float d = length(uv);
    //d -= 0.5;
    //float c = smoothstep(0., .2, d);
    float c = .25 / d;
    c = pow(c, 1.6);
    fragColor = vec4(uv, 0., 1.);
    fragColor = vec4(vec3(c), 1.);
}